Atherosclerosis is a disease of the arteries responsible for coronary heart disease (CVD) that underlies most deaths in industrialized countries (Lusis, 2000). Several risk factors for CHD have now been well established: dyslipidemias, hypertension, diabetes, smoking, poor diet, inactivity and stress. The most clinically relevant and common dyslipidemias are characterized by an increase in beta-lipoproteins (VLDL and LDL particles) with hypercholesterolemia in the absence or presence of hypertriglyceridemia (Fredrickson et al, 1967). An isolated elevation of LDL cholesterol is one of the most common risk factors for CVD. Twin studies (Austin et al, 1987) and family data (Perusse, 1989; Rice et al, 1991) have shown the importance of genetic factors in the development of the disease, particularly when its complications occur early in life. Mendelian forms of hypercholesterolemia have been identified: at first the autosomal dominant form (ADH) (Khachadurian, 1964) and later the autosomal recessive form (ARH), initially described as “pseudohomozygous type II hyperlipoproteinemia” (Morganroth et al, 1967).
ADH is a heterogeneous genetic disorder. Its most frequent and archetypal form is Familial Hypercholesterolemia (FH) with a frequency of 1 in 500 for heterozygotes and 1 per million for homozygotes (Goldstein et al, 1973). The disease is co-dominant with homozygotes being affected earlier and more severely than heterozygotes. FH is caused by mutations in the gene that encodes the LDL receptor (Goldstein & Brown, 1978) (LDLR at 19p13.1-p13.3) (MIN 143890). It is characterized by a selective increase of LDL cholesterol levels in plasma giving rise to tendon and skin xanthomas, arcus corneae and cardiovascular deposits leading to progressive and premature atherosclerosis, CHD and mortality (occurring before 55 years). The second form of ADH is Familial Defective apo B-100 (FDB) caused by mutations in the apolipoprotein B gene (APOB at 2p23-p24), encoding the ligand of the LDL receptor (Inneraty et al, 1987) (MIN 144010). The existence of a greater level of genetic heterogeneity in ADH (Saint-Jore et al, 2000) has been reported and the implication of a third locus named HCHOLA3 (formerly FH3) has been detected and mapped at 1p34.1-p32 in a French family (Varret et al, 1999) (MIM 603776). These results were confirmed by Hunt et al. in a large Utah kindred (Hunt et al, 2000).
PCSK9, for Proprotein Convertase Subtilisin/Kexin type 9 (also referred to as HCHOLA3, NARC-1, or FH3) is a protease belonging to the proteinase K subfamily of the secretory subtilase family (Naureckiene et al., Arch. of Biochem. And Biophy., 420:55-57 (2003)). PCSK9 has been shown to play a role in cholesterol homeostasis by regulating apolipoprotein receptor secretion. It may also have a role in the differentiation of brain cortical neurons (Seidah et al., PNAS 100(3):928-933 (2003)).
The wild type PCSK9 gene contains 12 exons. The translated protein contains a signal peptide in the NH2-terminus, and in cells and tissues an about 74 kDa zymogen (precursor) form of the full-length protein is found in the endoplasmic reticulum. During initial processing in the cell, the about 14 kDa prodomain peptide is autocatalytically cleaved to yield a mature about 60 kDa protein containing the catalytic domain and a C-terminal domain often referred to as the cysteine-histidine rich domain (CHRD) (FIG. 1). This about 60 kDa form of PCSK9 is secreted from liver cells. The secreted form of PCSK9 appears to be the physiologically active species, although an intracellular functional role of the about 60 kDa form has not been ruled out.
Several mutant forms of PCSK9 are known, including S127R, N157K, F216L, R218S, and D374Y, with S127R, F216L, and D374Y being linked to autosomal dominant hypercholesterolemia (ADH). Benjannet et al. (J. Biol. Chem., 279(47):48865-48875 (2004)) demonstrated that the S127R and D374Y mutations result in a significant decrease in the level of pro-PCSK9 processed in the ER to form the active secreted zymogen. As a consequence it is believed that wild-type PCSK9 increases the turnover rate of the LDL receptor causing inhibition of LDL clearance (Maxwell et al., PNAS, 102(6):2069-2074 (2005); Benjannet et al., and Lalanne et al), while PCSK9 autosomal dominant mutations result in increased levels of LDLR, increased clearance of circulating LDL, and a corresponding decrease in plasma cholesterol levels (Rashid et al., PNAS, 102(15):5374-5379 (2005).
Lalanne et al. demonstrated that LDL catabolism was impaired and apolipoprotein B-containing lipoprotein synthesis was enhanced in two patients harboring S127R mutations in PCSK9 (J. Lipid Research, 46:1312-1319 (2005)). Sun et al. also provided evidence that mutant forms of PCSK9 are also the cause of unusually severe dominant hypercholesterolaemia as a consequence of its affect of increasing apolipoprotein B secretion (Sun et al, Hum. Mol. Genet., 14(9):1161-1169 (2005)). These results were consistent with earlier results which demonstrated adenovirus-mediated overexpression of PCSK9 in mice results in severe hypercholesteromia due to durastic decreases in the amount of LDL receptor Dubuc et al., Thromb. Vasc. Biol., 24:1454-1459 (2004), in addition to results demonstrating mutant forms of PCSK9 also reduce the level of LDL receptor (Park et al., J. Biol. Chem., 279:50630-50638 (2004). The overexpression of PCSK9 in cell lines, including liver-derived cells, and in livers of mice in vivo, results in a pronounced reduction in LDLR protein levels and LDLR functional activity without changes in LDLR mRNA level (Maxwell K. N., Breslow J. L., Proc. Nat. Amer. Sci., 101:7100-7105 (2004); Benjannet S. et al., J. Bio. Chem. 279: 48865-48875 (2004)).
Using the above examples, it is clear the availability of novel forms of PCSK9 provide an opportunity for the identification of PCSK9 agonists, as well as, in the identification of PCSK9 inhibitors. All of which might be therapeutically useful under different circumstances.
The present invention also relates to recombinant vectors, which include the isolated nucleic acid molecules of the present invention, and to host cells containing the recombinant vectors, as well as to methods of making such vectors and host cells, in addition to their use in the production of PCSK9b and PCSK9c polypeptides or peptides using recombinant techniques. Synthetic methods for producing the polypeptides and polynucleotides of the present invention are provided. Also provided are diagnostic methods for detecting diseases, disorders, and/or conditions related to the PCSK9b and PCSK9c polypeptides and polynucleotides, and therapeutic methods for treating such diseases, disorders, and/or conditions. The invention further relates to screening methods for identifying binding partners of the polypeptides.